The feasibility of using digital fly-by-wire systems to control aircraft was demonstrated by developing and flight testing a single channel system, which used Apollo hardware, in an F-8C test airplane. This is the first airplane to fly with a digital fly-by-wire system as its primary means of control and with no mechanical reversion capability. The development and flight test of a triplex digital fly-by-wire system, which will serve as an experimental prototype for future operational digital fly-by-wire systems, is underway

Jarvis, C. R.

English

NASA Technical Reports Server

1. AN OVERVIEW OF NASA's DIGITAL FLY -BY -WIRE 
TECHNOLOGY DEVELOPMENT PROGRAM 
Calvin R . Jarvis 
NASA Flight Research Center 
SUMMARY 
The feasibility of using digital fly-by-wire systems to control aircraft was 
demonstrated by developing and flight testing a single channel system, which used 
Apollo hardware, in an F-8C test airplane. This is the first airplane to fly with a 
digital fly-by-wire system as its primary means of control and with no mechanical 
reversion capability. The development and flight test of a triplex digital fly-by- 
wire system, which will  serve as  an experimental prototype for future operational 
digital fly-by-wire systems, is underway. 
INTRODUCTION 
The advantages of digital fly-by-wire (DFBW) systems in terms of control 
system flexibility and reliability were demonstrated for spacecraft applications in 
NASA's manned space program. However , the transfer of this technology from 
spacecraft to aircraft is not direct and will require the identification and solution 
of many problems. 
DFBW technology, when fully utilized in the flight control system of an air- 
craft, can provide significant advantages over conventional control systems in 
terms of reduced costs , weight, and volume, and in improved performance. A 
redundant digital system, which can identify in-flight system failures and recon- 
figure itself, offers a potential reliability comparable to that of the basic aircraft 
structure as well as  the advantages of automatic control techniques. 
Although these benefits cannot be easily quantified for all classes of aircraft , 
design studies do indicate major rewards in terms of more effective flight control 
systems and, thus,  more effective aircraft. But even more important, these sys- 
tems lay the ground work for active control technology, and it is the active- 
control-configured aircraft that *offers the greatest potential in economic gains and 
performance advancements. 
https://ntrs.nasa.gov/search.jsp?R=19750010174 2020-03-22T22:22:55+00:00Z
The overall objective of NASA's digital fly-by-wire program is to provide the 
foundation for this technology, in terms of design criteria and operational experi- 
ence, which will lead to the development of practical digital fly-by-wire systems 
for future aircraft. To accomplish this objective, the program was separated into 
two phases, with an F-8C airplane (fig. 1) used as the test vehicle. 
The goal of Phase I, which has been accomplished, was to demonstrate the 
feasibility of using a DFBW system as the primary flight control system of an air- 
craft. To accomplish this goal, a single channel DFBW primary flight control sys- 
tem was flight tested, using an analog backup control system for fail/safe redun- 
dancy. 
The goal of Phase I1 is to establish a design base for the development of prac- 
tical DFBW systems. This will involve the development and flight test of a triplex 
DFBW system using redundancy management and data bus concepts. 
Figure 2 shows the schedule for Phases I and 11. The major aspects of each 
phase are discussed in the following sections. 
SINGLE CHANNEL SYSTEM DEVELOPMENT 
To establish the feasibility of the DFBW concept, a system was designed to 
replace the basic mechanical primary flight control system of the F-8C test airplane 
in all three control axes. All mechanical connections linking the pilot's control 
stick and rudder pedals to the control surfaces were removed. To be compatible 
with fly-by-wire design philosophy and development practice no mechanical 
reversion capability was provided even during the first part of the flight-test pro- 
gram. This is particularly significant because it required that satisfactory design 
and test techniques be demonstrated before the first flight. A single channel 
digital system concept was selected as the most straightforward approach to estab- 
lishing system feasibility. 
To minimize cost and development time, digital hardware and software orig- 
inally developed for the Apollo program were used as the heart of the digital sys- 
tem. An Apollo guidance and navigation system was used which consisted of a 
digital guidance computer, an inertial measurement unit, and associated interface 
elements. Use of this hardware also made available highly trained Apollo support 
teams. Another factor leading to the selection of the Apollo computer was its 
demonstrated 70,000-hour mean-time-before-failure record. This factor overrode 
shortcomings of the hardware which resulted in some operational constraints. 
A more complete description and discussion of the digital system is presented 
in paper 2 .  Pertinent aspects of man-rated software are covered in paper 4 .  
To provide redundancy if the primary digital system failed an analog flight 
control system f r o m  a lifting body research vehicle was modified extensively and 
installed in the F-8C airplane' as a triplex analog backup control system (paper 3 )  . 
2 
Phase I began in January 1971. During the following 1 5  months, five major 
contractors took part in the development and flight qualification of the Phase I sys- 
tem. These contractors and their areas of responsibility were: 
Delco Electronics . . Digital system hardware 
The Charles Stark Draper . . Digital system software 
Laboratory Inc . (MIT) 
Sperry Flight Systems Division . . Analog backup control system 
Hydraulic Research and . . Secondary actuators 
Manufacturing Company 
Ling-Temco-Vought Inc . . . Aircraft and electrical systems 
In addition to control law design and contractor coordination, NASA was 
responsible for specifying the Phase I system baseline configuration and interface 
requirements verifying the final software and hardware flight readiness and con- 
ducting the flight tests. 
The Phase I system was first used in flight on May 25, 1972 .  This was the first 
flight of an aircraft in which a digital fly-by-wire flight control system was the 
primary means of control. As  noted previously, no mechanical reversion capability 
was provided. Confidence in the reliability of the digital system was demonstrated 
by using it on the first takeoff and landing. 
Forty-two flights were made before the flight program was completed in 
November 1973.  The total flight time was 58 hours. The pilot controlled the air- 
plane most of this time using the primary digital system. Approximately 1 4  hours 
were flown using the analog backup system for evaluation purposes, inasmuch as no 
digital system failures were experienced during flight. The flight-test results are 
presented in papers 3 and 6.  
Phase I established the feasibility of DFBW systems for primary aircraft control 
and provided flight data related to control law design, software verification and 
operational procedures for DFBW systems. 
MULTICHANNEL SYSTEM DEVELOPMENT 
The goal of Phase I1 is to establish a design base for the development and imple- 
mentation of future practical DFBW systems. To accomplish this goal a multi- 
channel system is being developed which will provide redundancy management 
flight-test experience and verify other concepts of particular concern to the space 
shuttle orbiter development. 
The Phase I1 system configuration and major tasks are discussed in the follow- 
ing sections. 
3 
System Configuration 
A simplified diagram of the fully redundant triplex DFBW system is shown in 
figure 3 .  The principal elements of the system are to be installed on a removable 
pallet assembly, a s  the single channel system was  in Phase I .  Major components 
developed for Phase I such as  the analog backup control system redundant 
secondary actuators electrical power system and instrumentation system , are to 
be retained for use during Phase 11. 
Dedicated, redundant sensors will be used to measure airplane angular rate, 
attitude acceleration and air data. Sensor inputs will be cross-strapped to each 
computer and synchronized on a bit-by-bit basis. Surface command outputs will 
be voted for fault detection and supplied to the triplex, force-summed, secondary 
actuator servo valves. Differential pressure equalization will  be used to minimize 
nonlinear secondary actuator effects. A two channel (active and monitor) analog 
backup control system will be provided for use if the primary system fails. 
The system will  be designed to minimize ground operational and preflight 
support requirements. All system status testing will be automated and will be done 
onboard the airplane. 
The digital processor selected for Phase I1 is a state-of-the-art off-the-shelf 
general-purpose computer with floating-point and microprograming features. The 
computer is an order of magnitude faster than the Apollo computer used in Phase I .  
The main storage memory is fully programable which provides greater software 
flexibility. This increase in computer capability is of particular importance in 
carrying out the objectives of the Phase I1 program. 
Evaluation of Space Shuttle Orbiter DFBW Concepts 
An important aspect of Phase I1 is coordination with the shuttle orbiter flight 
control system development. In addition to being the first application of DFBW in 
an aerodynamic vehicle, the orbiter will Contribute significantly to digital system 
technology by addressing the problems of redundancy management (reliability) 
and overall mechanization. 
The shuttle flight control system will use the same digital processors as those 
being used in Phase I1 of the F-8 DFBW program. The Phase I1 triplex processor/ 
sensor configuration will thus make it possible to evaluate certain aspects of the 
shuttle system by using the F-8C airplane as  a test-bed. 
Redundancy management. - The redundancy management concept developed 
for the orbiter system to detect and isolate digital processor and control system 
sensor failures will be implemented and flight tested during Phase 11. Because a 
reliable means of achieving failure detection and isolation is a major problem in the 
design of redundant DFBW systems flight-test verification of the concepts in 
Phase I1 will  establish a significant data base for future applications. 
Data bus.  - The data bus concept of reducing cabling and connector require- 
ments for redundant systems by compressing data from several sensors onto 
4 
redundant transmission lines is important in the development of FBW technology. 
The discrete format of signals in DFBW systems makes the data bus a natural solu- 
tion to the complex cabling problem. In Phase I1 the technique proposed for the 
shuttle system will  be used to process trim commands and mode panel in 
(e g a 
the airplane cockpit to the palletized system in the equipment bay. This will  
greatly reduce the number of wires and will  verify data bus utility for shuttle as 
well as future system applications. 
status lights mode change commands) and to transmit the information from 
Computer synchronization. - Of major concern in the design of any redundant 
DFBW system is whether or not to synchronize the computer operations and, if so, 
the best way to do it The Phase I1 system wil l  be designed with enough flexibility 
to permit the use of various synchronization approaches as well as asynchronous 
operation. Included will  be the baseline approach selected for the orbiter system. 
Control laws. - The first control laws to be evaluated in flight during Phase I1 
will be similar to those developed for the F-86 airplane during Phase I and similar 
in format to those being developed for the shuttle orbiter. These include C* and 
rate command modes for pitch and roll as well as direct control modes for each axis. 
Control law software required for moding and initialization will  therefore be similar 
for both programs, which will permit some system verification. 
Higher order programing language. - A higher order programing language, 
called Higher Aerospace Language (HAL), is being developed in support of shuttle 
software requirements. Use of this language in developing certain elements of the 
control laws for the Phase I1 system will  make it possible to debug and verify it 
before it is actually applied to the shuttle orbiter. 
Backup control system. - The present shuttle system configuration wil l  require 
a dissimilar single channel digital backup control system during initial horizontal 
flight tests to override possible primary system generic failures. The executive 
structure for the shuttle backup system will  be implemented in the Phase I1 system 
and flight-qualified through flight-test verification. 
Advanced Control Law Development 
To assess the capability of a digital system to perform the functions necessary 
for future active control applications, additional control laws will be programed and 
evaluated during Phase 11. A specific task is the investigation of improvements that 
can be made in aircraft control law implementation as a result of the rapidly advanc- 
ing digital fly-by-wire system capability. The availability of a powerful onboard 
digital computer system that can process sophisticated flight control laws in real 
time has added a new dimension to realizable control law development. Control 
laws previously too complex and unwieldy for analog system applications can now 
be considered prime candidates for digital applications. 
Initial Phase I1 control law research is directed toward the use of active control 
for maneuver load control, possible improvement in ride quality, suppression of 
turbulence effects flight envelope limiting techniques, and operation at reduced 
5 
static-stability margins. The basic elements of such a control law now being devel- 
oped for the longitudinal axis are illustrated in figure 4 .  The structure consists of 
a boundary controller for angle-of-attack limiting, a normal controller for longi- 
tudinal commands a direct-lift controller for commanding symmetric ailerons, a 
load controller , and autopilot modes. A proportional flap-to-elevator crossfeed is 
planned to compensate for the pitching moment produced by symmetric aileron 
deflection. 
The design objective for the longitudinal axis is to achieve good handling 
qualities by matching desired response criteria for both positive and negative static 
stability margins. Gust load alleviation is provided by additional damping of short- 
period dynamics using the elevator surface. Angle-of-attack limiting is provided 
throughout the flight envelope. 
Direct lift of the symmetric ailerons is combined with the elevator to minimize 
drag during maneuvers and to enhance gust load alleviation during cruise. The 
three autopilot modes are the conventional attitude hold, altitude hold, and Mach 
hold. 
Other advanced control law prospects, in which adaptive techniques and optimal 
control theory are used are being studied for possible flight-test evaluation during 
Phase 11. 
Remotely Augmented Vehicle Facility 
A s  part of Phase I1 a unique remotely augmented vehicle facility is to be 
developed to support advanced control law research and flight-test evaluation. 
A diagram of the proposed facility is shown in figure 5 .  During a test flight a 
special remotely augmented vehicle test mode may be selected by the pilot that will 
divert his control commands to a ground computer facility via a telemetry down- 
link y on which a particular advanced control law to be evaluated is programed. 
Control surface commands are determined by the ground computer on the basis of 
the pilot's airborne commands, the airplane's response, and the programed control 
law. The surface commands are  then transmitted, via a telemetry up-link, to the air- 
plane system and the corresponding control surface. The pilot flies the airplane 
through the control laws programed on the remotely located ground computer. Fail 
safety will be maintained through the use of reasonability tests built into the ground 
computer facility and safety networks in the telemetry equipment. This approach 
will  permit a great deal of flexibility for control law evaluation without compro- 
mising the basic airborne system verification requirements. 
CONCLUDING REMARKS 
The full realization of the benefits of active control technology and the benefits 
predicted by its application to aircraft design depends on the development of prac- 
tical, reliable, and versatile .digital fly-by-wire (DFBW) control systems. The 
feasibility of such systems and confidence in their reliability and integrity were 
6 
established in Phase I of the F-8 DFBW program. The goal of Phase I1 of the pro- 
gram is to establish a design base from which practical, reliable systems can be 
developed. This will  be accomplished by developing and flight testing a fully 
redundant triplex DFBW system. 
The multichannel system development carried out during Phase I1 will establish 
techniques for validating redundant system software and hardware interfaces and 
for establishing operating procedures unique to DFBW systems. Flight-test eval- 
uation of orbiter control system concepts using the F-8C airplane will  result in 
verification of redundancy management software for digital processor and sensor 
fault detection and reduced generic failure probabilities for the orbiter system. 
Flight-test evaluation of advanced control laws during Phase I1 will  provide an 
opportunity to assess the capability of DFBW systems to perform the complex control 
tasks associated with active control applications. 
The NASA DFBW program, although complementary to other fly-by-wire 
activities, is aimed specifically at providing the technology for practical digital 
flight controls for civil aircraft. A s  such, it represents the first step toward a new 
generation of active-control-configured aircraft which will offer significant economic 
advantages. 
7 
Phase I c Single channel system 
Phase I I  
Multichannel system 
I 
Figure 1. F-8C test airplane. 
Calendar year 
1-1 Development 
Flight test 
Figure 2 .  F-8 DFBW program schedule. 
8 
Primary system 
Secondary actuator 
Figure 3 .  Phase I1 system configuration. 
9 
Bank angle Autopilot modes 
P i tch  at t i tude 
Mach number  Al t i tude hold 
Al t i tude 
Ang l e-of-attack 1 i mit  Boundary control ler  
Angle of attack 
I Pi tch  ra te  -4 I i miter 
Reduced static stabil ity 
I 
To elevator actuators +------ 
I 
Maneuver load control  
Gust load alleviation 
To ai leron actuators 
Normal acceleration 
Figure 4 .  Active control law diagram for longitudinal axis. 
10 
Figure 5 .  Remotely augmented vehicle facility. 
11 


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